1. Field of the Invention
This invention relates to films and coatings having anisotropic conductive pathways therein, and to methods for making the films and coatings, and to electronic components having such a coating thereon. The invention is particularly for use in interconnection technology in the electronics industry.
2. Description of Related Art
Electronic components such as semiconductor chips, circuit boards, flex connectors and displays often have very small connectors such as pad, pins and leads and have minimal gaps (pitch) between connectors. Conventional solder may give rise to difficulties because the solder may bridge the gap between two adjacent connectors on the same component. Therefore anisotropically-conductive adhesives have been proposed for electrical interconnection. An anisotropically conductive adhesive (ACA) conducts electricity in one direction only (usually denoted as the Z direction) and should eliminate conduction in the plane perpendicular thereto (the X and Y directions).
Various proposals for ACA's are reviewed by Ogunjimi et al in Journal of Electronics Manufacturing (1992) 2, 109-118 and are described in U.S. Pat. No. 4,740,657 Tsukagoshi et al; U.S. Pat. No. 3,359,145 Salyer et al; U.S. Pat. No. 4,548,862 Hartman; U.S. Pat. No. 4,644,101 Jin et al; U.S. Pat. No. 4,170,677 Hutcheson and U.S. Pat. No. 4,737,112 Jin et al.
Jin et al in IEEF Trans. on Components, Hybrids and Manufacturing Technology, Vol. 16, (8), 1993, p. 972 (the contents of which are incorporated herein by reference) describes anisotropically conductive films consisting of a single layer of magnetically separated conductor spheres in a polymer matrix. In a vertical magnetic field, ferromagnetic spheres in a viscous medium become parallel magnetic dipoles, and repel one another to produce a two-dimensional particle distribution which is described as uniform. This structure is then frozen in by cooling or curing of the polymer matrix, which may be an elastomer or an epoxy or thermoplastic adhesive.
ACAs rendered anisotropic by application of a magnetic field have not been adopted commercially, so far as the present Applicants are aware. The proposals by Jin et al (which date back to 1986, as shown by U.S. Pat. No. 4,737,112) require specialized particles which are both magnetic and electrically conductive. Magnetic particles which have been distributed by a magnetic field may form undesirable dendritic protrusions, which can only be circumvented by balancing magnetic force on the particles with surface tension of the polymer and gravity effects. Such dendritic structures are undesirable for interconnection in the electronics field, where the distribution of conductive pathways is critical. Ordering magnetic particles in a regular adhesive matrix therefore imposes restraints with regard to particle type (ferromagnetic), field strength and matrix properties. Ferromagnetic conductive particles such as Fe, Ni, and Co tend to have high densities, low compressibility, are prone to settling out of formulations and are either toxic or easily corroded. It is therefore undesirable to use them as interconnection particles.
Hogerton in Journal of Electronics Manufacturing (1993) 3, 191-197 (the contents of which are incorporated herein by reference) discusses the status of 3M's adhesive interconnection technology, with particular emphasis on anisotropically conductive adhesive films. Hogerton indicates that a new film construction will avoid the inherent limitations of random dispersion of conductive particles and will provide for direct adhesive flip-chip attachment of unbumped integrated circuits. However the new film construction is not disclosed.
In an unrelated area of technology, it is known to make a magnetic liquid or "ferrofluid" consisting of a colloidal suspension of minute ferromagnetic particles in a non-magnetic carrier liquid. A typical ferrofluid may consist of magnetite particles (Fe.sub.3 O.sub.4) having a particle size in the range 2 nanometres to 0.1 micrometres (and a mean size of about 0.01 micrometres) in kerosene as carrier liquid with a surfactant to prevent agglomeration of the particles (see Skjeltorp "One- and Two-Dimensional Crystallization of Magnetic Holes" in Physical Review Letters, Volume 51, Number 25, Dec. 19, 1983, 2306-2309, Skjeltorp A. T. and Helgesen, G. Physica A, 176, 37, 1991; Skjeltorp A. T. J. Appl. Physics 57(1), 3285, 1985); and U.S. Pat. No. 4,846,988 Skjeltorp, the contents of which are incorporated by reference.
U.S. Pat. No. 5,075,034 Wanthal describes a two component adhesive composition which is curable by induction heating (i.e. with an induced magnetic field) and which contains conductive carbon black along with iron oxide particles. However there is no suggestion that the iron oxide particles may be of such small particle size as to form a colloidal suspension. This patent therefore does not relate to the field of ferrofluids or of anisotropically conductive adhesives.
EP 0 208 391 A2 Ferrofluidics Corporation describes an electrically conductive ferrofluid composition which contains carbon particles having diameters of about 5 to 30 nm. The composition is intended for use in a ferrofluid exclusion seal apparatus for sealing of computer disk drive spindles.
JP 3095 298 Nippon Seiko KK describes a magnetic fluid composition containing fine ferromagnetic particles and fine particles of at least one metal, alloy or electrically conductive ceramic as a material which imparts electrical conductivity. The conductive particles have a diameter distributed within the range of a few nm to a few hundred nm while in the case of anisotropic particles the length of the longer particles may be a few tens of nm.
In a further unrelated area of technology, U.S. Pat. No. 4,946,613 Ishikawa describes a photosetting ferrofluid for use in magnetic flaw detection or for visualizing magnetically recorded patterns. The photosetting ferrofluid comprises a carrier, a ferrofluid in which the ferromagnetic particles have an adsorbed surfactant (or the surfactant is dispersed in the carrier) and a photosetting resin. The photosetting resin may be the carrier. The ferrofluid is applied to a surface to be analyzed and is then subjected to a magnetic field. The applied ferrofluid will be attracted to the portion where the magnetic flux leaks i.e. to cracks or defects in the surface, and will swell to form a pattern corresponding to the configuration of the defect portion. A beam of light is then used to set or harden the photosetting resin so as to fix the defect pattern thus formed.
Ishikawa does not envisage the application of a magnetic field to create a chosen alignment of particles, followed by fixation of this alignment.
In International Patent Publication No. WO 95/20820 (hereafter called "the parent application", the contents of which are incorporated herein by reference) published after the priority date of this application, we have described a composition comprising: (i) a ferrofluid comprising a colloidal suspension of ferromagnetic particles in a non-magnetic carrier liquid, and (ii) a plurality of electrically-conductive particles having substantially uniform sizes and shapes, dispersed in the ferrofluid.
Preferably the average particle size of the electrically conductive particles is at least 10 times (and more particularly 100 times, most preferably 500 times) that of the colloidal ferromagnetic particles. The non-magnetic carrier liquid may be curable or non-curable and may be selected from:
(i) a curable liquid composition, PA1 (ii) a mixture of a curable liquid composition and a liquid carrier in which the ferromagnetic particles have been suspended, and PA1 (iii) a non-curable carrier liquid, PA1 (a) applying to one set of conductors a layer of an adhesive composition comprising a curable composition as described above; PA1 (b) bringing a second set of conductors against the layer of adhesive composition; PA1 (c) exposing the layer of adhesive composition to a substantially uniform magnetic field such that interaction between the ferrofluid and the electrically-conductive particles causes the electrically-conductive particles to form a regular pattern of particles each in electrical contact with an adjacent particle and/or with a conductor in one or both sets whereby conductive pathways are provided from one set of conductors to the other set, each pathway comprising one or more of the electrically-conductive particles; and PA1 (d) curing the composition to lock the pattern in position and to bond the conductors. PA1 (a) applying to one set of conductors a layer of a non-curable composition as described above wherein the electrically-conductive particles have a latent adhesive property; PA1 (b) bringing a second set of conductors against the layer of the composition; PA1 (c) exposing the layer of the composition to a substantially uniform magnetic field such that interaction between the ferrofluid and the electrically-conductive particles causes the electrically-conductive particles to form a regular pattern of particles each in contact with an adjacent particle and/or with a conductor of one or both sets; and PA1 (d) activating the latent adhesive property of the particles whereby conductive pathways are provided from one set of conductors to the other set, each pathway comprising one or more of the electrically-conductive particles, and the conductors are bonded by the particles. PA1 (i) a solidifiable ferrofluid composition, the ferrofluid comprising a colloidal suspension of ferromagnetic particles in a non-magnetic carrier, and PA1 (ii) a plurality of electrically-conductive particles, dispersed in the ferrofluid, PA1 (a) applying to a substrate, a layer of a composition comprising: PA1 (b) exposing the liquid composition to a magnetic field to array the electrically-conductive particles in a non-random pattern, and PA1 (c) concurrent with or subsequent to step (b), exposing the composition to solidifying conditions for the composition, and PA1 (d) optionally removing the layer of solid composition from the substrate to form a film. PA1 (1) a colloidal dispersion of ferromagnetic particles in a solidifiable liquid composition (i.e the solidifiable composition acts as the carrier of the ferrofluid), or PA1 (2) a mixture of a solidifiable liquid composition and a colloidal dispersion of ferromagnetic particles in a liquid carrier.
but if the carrier liquid is non-curable and the curable liquid composition is not present, the electrically-conductive particles have a latent adhesive property.
In the parent application we have also described a method of making an anisotropically-conductive bond between two sets of conductors, comprising:
The parent application further describes a method of making an anisotropically conductive bond between two sets of conductors, comprising:
In a preferred feature of the invention of the parent application, pressure is applied to urge the respective sets of conductors towards one another before and/or during the curing step or the activation of the latent adhesive property.